Some futurologists move in academic circles while others ply their "futuring" trade in business or politics, and still others are simply interested amateurs.

Predictions tend to fall short in a few key areas. Context often eludes futurologists, who color their forecasts with their experiences of the present and recent past, which can't account for shifts in social attitudes, economic forces or political realities yet to come. And then there are those inventions that defy prediction -- the kind that break the rigid cause-and-effect chains upon which many predictions hang.

Take the aforementioned "Limits to Growth," which vastly overestimated the depletion of oil, natural gas, silver, uranium, aluminum, copper, lead and zinc. It belongs to a tradition of doom-and-gloom scenarios extending back to English scholar Thomas Malthus. In 1798, Malthus predicted that exponential population growth would soon outstrip more gradual increases in food production. Along similar lines, British economist William Stanley Jevons made his fame with "The Coal Question" (1865) by predicting that Britain would run out of coal within a few short years. The U.S. Department of the Interior announced in 1939 -- and again in 1951 -- that America had only 13 years' worth of oil left in the tank [source: The Economist].

Though wrong -- often because they underestimated or ignored changes in known reserves, economic forces or technology -- many of these ideas and their underlying arguments are still quoted by pundits, environmentalists and high school debate teams [sources: The Economist; Simon].

Conversely, Moore's law, which predicts that the number of transistors on integrated circuits in computers doubles every two years, has held up better over time, in part because it assumes technological innovation -- and in part because Moore revised his timescale [sources: Kanellos; Moore].

The 1970s and 1980s saw a flowering of famous futurologist books, including "The Coming of Post-Industrial Society" (1973) by Daniel Bell, "The Fate of the Earth" (1982) by Jonathan Schell and "Green Machines" (1986) by Nigel Calder. Some include Alvin Toffler's "Future Shock" (1970) among this group, while others consider it a work of sociology.

Today, many technology forecasters have a fiduciary stake in their predictions. One such person, Paul Saffo of the Silicon Valley investment research firm Discern, bases his outlook on four indicators: contradictions, inversions, oddities and coincidences. Others use the following strategies [source: Pearlstein]: